U.S. Pat. No. 4,405,792 (Walter I) discloses 4-(3-chlorocyclohex-3-enyl)pyridine, 4-(4-chlorocyclohex-3-enyl)pyridine, a process for making these compounds by a Diels-Alder reaction between chloroprene and a 4-vinylpyridine, and a process for converting them to 4-(3-chlorophenyl)pyridine and 4-(4-chlorophenyl)pyridine by catalytic dehydrogenation.
Copending application Ser. No. 495,977, filed May 19, 1983, now U.S. Pat. No. 4,533,735, issued Aug. 8, 1985, in the name of Thomas J. Walter (Walter II) discloses processes by which 4-(4-halophenyl)pyridines, such as the 4-(4-chlorophenyl)pyridine produced in Walter I, can be converted to 4-(4-halo-3-nitrophenyl)pyridines, then to 4-(3-aminophenyl)pyridines, and ultimately to the antibacterial 1-alkyl-1,4-dihydro-4-oxo-7-pyridinyl-3-quinolinecarboxylic acids of Sterling Drug's U.S. Pat. Nos. 3,753,993 (Lesher et al.), 3,907,808 (Lesher and Catabateas), and 4,118,557 (Lesher).
Copending application Ser. No. 497,026, filed May 23, 1983, now U.S. Pat. 4,552,963, issued Nov. 12, 1985, in the name of V. Ramachandran (Ramachandran I) discloses improved processes for preparing 4-(halocyclohex-3-enyl)pyridines and derivatives wherein a boron trifluoride catalyst is employed directly to improve the yield of 4-(4-halocyclohex-3-enyl)pyridine and indirectly to improve the yields of derivatives thereof.
Copending application Ser. No. 497,027, filed May 23, 1983, now U.S. Pat. No. 4,550,167, issued Oct. 29, 1985, in the name of V. Ramachandran (Ramachandran II) discloses 4-(halocyclohex-3-enyl)pyridine salts which may be prepared by reacting 4-(halocyclohex-3-enyl)pyridines with relatively strong acids to facilitate isolating and/or aromatizing the 4-(halocyclohex-3-enyl)pyridines.
Separately and in combination, the aforementioned references disclose useful processes for preparing antibacterial agents and intermediates therefor. However, since the aromatization techniques taught in these references are not particularly efficient, there is still room for improvements in these processes. Also, it would be desirable to discover an aromatization technique that would not only permit the efficient aromatization of the difficulty aromatizable 4-(halocyclohex-3-enyl)pyridines of Walter and Ramachandran but also present an alternative method of aromatizing the more easily aromatizable 4-(cyclohex-3-enyl)pyridines having no halo substituents on the cyclohexene ring.
As described in March, Advanced Organic Chemistry, McGraw Hill (New York), 1977, pages 1077-1078, it is known that cyclohexenes can be aromatized in a variety of ways in addition to the ways taught by Walter and Ramachandran. However, it is also known that these other conventional techniques are sometimes unsatisfactory for the aromatization of particular cyclohexenes, and drastic conditions are occasionally required for the aromatization. For example, Newman et al., Journal of the American Chemical Society, Vol. 63, 1941, pp. 1542-1544, disclose the inadequacy of various aromatization techniques in attempted aromatizations of 3-methyl-1,2,3,6-tetrahydrophthalic anhydride.
Newman et al. teach that their 3-methyl-1,2,3,6-tetrahydrophthalic anhydride can be aromatized by heating it with bromine in acetic acid and pyrolyzing the bromine-containing intermediate thus obtained. It would be desirable to be able to modify this brominative aromatization to make it useful for the aromatization of cyclohexenylpyridines.